The printed circuit board industry commonly uses ammoniacal alkaline copper etchant to remove unwanted copper from printed circuit boards as part of the fabrication process. The ammoniacal alkaline copper etchant is a mixture of copper ammonium chloride, ammonium chloride, ammonium hydroxide, ammonium carbonate, and small amounts of other materials. The copper ammonium chloride itself is the active etchant when the copper is in the cupric (+2) state. Cupric ammonium chloride attacks and dissolves metallic copper, forming cuprous (+1) ammonium chloride. The cuprous salt is inactive as an etchant material. Cuprous salts are reoxidized to the active etchant or cupric form by atmospheric oxygen.
This etchant is almost universally used in printed circuit board production shops. The etch rate is very fast and the etch solution can hold large amounts of copper. The normal maximum loading of copper is 105-150 grams of copper per liter (14-20 ounces of copper per gallon). The solution, once loaded with copper, is not discarded. It is recycled and processed to remove the excess copper to yield fresh etchant and metallic copper.
The process which is used for ammoniacal copper etchant regeneration in commercial recycling plants is complex and expensive. The spent etchant is contacted with a liquid ion exchange (LIX) material which is dissolved in a water immiscible organic solvent such as kerosene. This is normally a continuous process using countercurrent flow apparatus. The copper-loaded LIX/kerosene mixture is contacted with a sulfuric acid solution, also using countercurrent flow apparatus. The sulfuric acid extracts the copper from the LIX/kerosene mixture to regenerate the ion exchange material. The copper sulfate/sulfuric acid solution is used to produce low value copper sulfate crystals. Alternatively, the copper sulfate/sulfuric acid solution can be electrolyzed in an electrolytic plating cell to recover higher value metallic copper.
The amounts of spent alkaline etchant produced are very large. Typically one gallon of spent etchant is produced for every 7 to 10 square feet of double sided printed circuit material processed. Even a moderately large shop can produce over one hundred thousand gallons of spent etchant per year. Because the quantities of used ammoniacal etchant are very large and reclaim is very complicated, the used etchant is shipped off-site to recycling facilities. These large shipments of etchant are expensive and hazardous, affording numerous opportunities for hazardous materials spills.
Commercial alkaline etchant recycling facilities are very large and complex. They have multiple large countercurrent extraction flow towers containing large volumes of recirculating etchant, kerosene, hazardous organic complexing agents, sulfuric acid, and copper sulfate. All of these materials are toxic and hazardous in the event of a plant accident and chemical spill. The kerosene solution is also combustible and presents a continuous fire hazard. If copper reclaim is done by electroplating, very large rectifiers with high power consumption are needed.
Another known process for ammoniacal etchant purification uses a special electrolytic cell attached to the etch machine to remove the copper. This has stringent technical and chemical design limits. A two cell process with a membrane separator is often used. Direct electrolysis of ammoniacal copper etchant is not practical due to the presence of chloride, which gives chlorine gas on electrolysis. The etchant was chemically changed from a chloride based to a sulfate based system. This uses copper ammonium sulfate instead of copper ammonium chloride as the active, but much slower etchant. The slower etchant was also needed due to design limits on the speed of electrolytic recovery in this in-plant system to maintain the correct copper concentration for reproducible etching. The actual etching rate is about three times slower than with copper chloride based, ammoniacal alkaline copper etchants. Most printed circuit shops are at or near capacity on their ammoniacal etcher, often fully using them two or even three shifts a day. Thus they would have to triple their capital investment in expensive machines to use this process.
A new process has been developed using metallic aluminum to remove the copper in a simple, one step reaction without the introduction of detrimental impurities and without the use of expensive membrane separators and rectifiers. This process is highly exothermic and difficult to control. An improved process using aluminum as the reductant has now been developed, which gives simple control of the copper reduction reaction.